Water is frequently used to transport unwanted materials—waste—to a facility where the waste is removed or neutralized in the water. For example, water carries most sewage and industrial waste, such as chemicals, in the form of wastewater to a treatment facility where the water is treated and then returned to the environment for future use. The wastewater treatment process typically includes three general phases. The first phase, or primary treatment, involves mechanically separating the dense solids in the wastewater from the less dense solids and liquid in the wastewater. This is typically done in sedimentation tanks with the help of gravity. The second phase, or secondary treatment, involves the biological conversion of carbonaceous and nutrient material in the wastewater to more environmentally friendly forms. This is typically done by promoting the consumption of the carbonaceous and nutrient material by bacteria and other types of beneficial organisms already present in the wastewater or mixed into the wastewater. The third phase, or tertiary treatment, involves removing the remaining pollutant material from the wastewater. This is typically done by filtration and/or the addition of chemicals and/or UV light and/or Ozone to neutralize harmful organisms and/or remove pollutant material.
The second phase of the wastewater treatment process typically includes an aerobic—with oxygen—portion in which bacterial and other microorganisms are provided dissolved oxygen to promote their consumption of the carbonaceous and nutrient materials, and an anoxic—oxygen from a nitrate/nitrite source—portion in which the bacteria and other microorganisms use the oxygen in the nitrate/nitrite for their metabolic functions. The second phase may also include an anaerobic—without oxygen—portion in which bacteria and other microorganisms metabolically function without oxygen. The aerobic, anoxic and anaerobic portions are typically carried out in tanks that are divided into aerobic, anoxic and anaerobic zones: The tank may include one zone in which the aerobic portion operates and one in which the anoxic portion operates and one in which the anaerobic portion operates, or the tank may include any combination of any number of these zones. In some applications, a tank may be solely dedicated to one of the three aerobic, anoxic and anaerobic portions.
In the aerobic process, wastewater that includes ammonium (NH4) and organic waste containing nitrogen, for example urea ((NH2)2CO), enters the aerobic zone. In the presence of dissolved oxygen (02), bacteria and other microorganisms convert the ammonium into nitrate (N03) via nitrite (N02). The nitrate can then be anoxically processed into nitrogen gas (N2), which is harmless in the environment. In the aerobic process, a blower and diffusers supply the dissolved oxygen to the wastewater. The blower provides air to the diffusers, and the diffusers generate and release tiny bubbles so that oxygen in the bubbles will dissolve in the wastewater. As the aerobic process progresses, the amount of ammonium in the wastewater decreases while the amount of nitrate and dissolved oxygen increases. The amount of dissolved oxygen increases because the demand for the dissolved oxygen decreases as the amount of nitrate increases. After most of the ammonium has been converted into nitrate, the wastewater is ready to be anoxically processed.
In the anoxic process, wastewater that includes nitrate and the organic waste containing nitrogen enters the anoxic zone. In the absence of dissolved oxygen, bacteria and other microorganisms convert the nitrate into nitrogen gas and the organic waste containing nitrogen into ammonium. As the anoxic process progresses, the amount of nitrate decreases and the amount of ammonium increases. After most of the nitrate has been converted into nitrogen gas, the wastewater is ready to be aerobically processed or treated in the tertiary treatment phase.
Mixing the contents in each of the aerobic and anoxic zones promotes the conversion reactions in each zone by increasing the contact of the components, such as the dissolved oxygen (aerobic zone), nitrite/nitrate (anoxic zone), wastewater, and bacteria and other microorganisms, with the other components in each zone.
Further to promote the conversion reactions in at least some of the zones, an Integrated Fixed-film Activating Sludge (IFAS) system that includes media may be employed. As typified by the AccuPac® cross-flow media product of Brentwood Industries, Inc. of Reading, Pa., such media are freely suspended in the wastewater and provide bacteria and other microorganisms a structure to hold onto. In other cases (such as the BioWeb™ media product from Entex, Inc. of Chapel Hill, N.C.), the IFAS may include a net or web that is anchored in the zone to provide bacteria and other microorganisms a structure to hold onto. In still other embodiments the IFAS may include both the net or web and the media. In any case, when IFAS is employed effectively in a treatment zone, the IFAS material is typically deployed widely throughout the zone to present a large surface area for biological conversion of the wastewater.
In the prior art, wastewater in the aerobic zone is typically mixed by the movement of the tiny bubbles injected by diffuser and further mixed by a mechanical mixer that includes a screw or blade that is turned by a motor. In the anoxic zone, the prior art typically employs a mechanical mixer alone for mixing, the tiny bubbles diffused in the aerobic stage being neither needed nor typically supplied in the anoxic stage.
The prior art means for mixing the wastewater in the aerobic and anoxic zones is subject to limitations and disadvantages. Mixing the aerobic zone with the movement of the tiny bubbles through the wastewater requires a substantial amount of tiny bubbles to be injected into the wastewater to sufficiently mix the wastewater. As the demand for dissolved oxygen diminishes with aerobic treatment over time, the amount of tiny bubbles that must be supplied to provide adequate mixing will greatly exceed the quantity of tiny bubbles required for sufficient oxygenation. Using the tiny bubbles for mixing when wastewater oxygen demand has diminished, the diffusers consume more power than required to oxygenate the wastewater and can inject more dissolved oxygen into the aerobic zone than required, in fact thereby potentially slowing the progress of the wastewater treatment.
Mixing the aerobic and anoxic zones with a mechanical mixer consumes a large amount of power relative to the amount of wastewater that it mixes, and often mixes some, but not all, of the wastewater in each zone. Thus, some of the sludge in the aerobic and anoxic zones remains on the bottom of the tank after it settles there. In the aerobic zone, the settled sludge can plug some of the diffusers. This can reduce the amount of dissolved oxygen injected into the wastewater, and thus requires one to clear the plugged diffusers.
In an improvement on the prior art, a tank for treating wastewater includes an aerobic zone in which bacteria and other microorganisms consume pollutants in the presence of dissolved oxygen, and an anoxic zone in which bacteria and other microorganisms convert pollutants in the absence of dissolved oxygen to a more environmentally friendly form. Advantageously, the tank also includes a mixer that generates large mixing bubbles, for example a bubble having a largest dimension of 6 inches to 10 feet. The mixing bubbles are large enough to move wastewater as they rise to the surface and generate a mixing current in the wastewater. The mixing current mixes the wastewater, and bacteria and other microorganisms to promote the bacteria and other microorganisms' conversion of the pollutants contained in the wastewater.
Because the mixing bubbles are large enough that the amount of oxygen that they release into the wastewater as they move through it is negligible, when large bubbles are used for mixing in the anoxic zone, the zone remains anoxic as the large bubbles from the mixer rise toward the surface of the wastewater. Accordingly, large bubbles can be used for advantageously for mixing in both the aerobic and anoxic zones.
It may appear even to those skilled in the art that the presence of IFAS in such a wastewater treatment system would present a physical barrier, whereby large mixing bubbles are broken up with the consequent loss of the advantages afforded by employing such bubbles for mixing. However, the inventors hereof have discovered that large bubbles may advantageously be used for mixing wastewater in both aerobic and anoxic zones even when IFAS is present.